Lincoln stole the Powhatan away from Mercer who was under original orders to fit out the Powhatan and join Fox at Charleston.
A sample of the telegrams below....
COMMANDANT NAVY YARD, Brooklyn, N. Y. Order of the President of the United States to the commandant navy yard, New York, regarding the fitting out of the U. S. S. Powhatan. EXECUTIVE MANSION, April 1, 1861. SIR: You will fit out the Powhatan without delay. Lieutenant Porter will relieve Captain Mercer in command of her. She is bound on secret service, and you will under no circumstances communicate to the Navy Department the fact that she is fitting out. Abraham Lincoln.
Official records of the Union and Confederate Navies in the War of the Rebellion. ; Series I - Volume 4: Operations in the Gulf of Mexico (November 15, 1860 - June 7, 1861); Operations on the Atlantic Coast (January 1, 1861 - May 13, 1861); Operations on the Potamac and Rappahannock Rivers (January 5, 1861 - December 7, 1861) Page 109 and 112.
APRIL 6, 1861. Give the Powhatan up to Captain Mercer. SEWARD. [Lieutenant D. D. PORTER.] [Telegram.]
APRIL 6, 1861. I received my orders from the President and shall proceed and execute them. D. D. PORTER. [Hon. WILLIAM H. SEWARD.]
These telegrams and letters are very numereous, as Mercer was confused and was trying to do his duty, kept trying to claim the Powhatan, and Porter, doing his duty, willingly carried out Lincoln's plans to make off with the Powhatan. Mercer even gave chase and caught up with the Powhatan at sea when he thought he had the right orders, but, Porter ignored the orders because Seward had signed them, not Lincoln. (see above). Fox and his fleet, on their way to Charleston, never knew the Powhatan was never going to meet them, and Fox's plan was completely foiled. Montgomery Blair, was the only cabinet member that wrote to Lincoln that Fort Sumter should be reenforced (all the others warned that only war would come of it and disagreed), Blair and his father was Lincoln's political confidant, so the late admission from Blair is very strong evidence.